Among pipe couplings of the type comprising a male pipe coupling member and a female pipe coupling member, there is well known a pipe coupling configured to detachably couple a male pipe coupling member and a female pipe coupling member by using a locking element(s), as disclosed, for example, in Patent Literature 1. A female pipe coupling member used in such a pipe coupling generally has a coupling body configured to receive a male pipe coupling member, a locking element radially displaceably retained in a locking element retaining hole of the coupling body, and a sleeve disposed around the outer peripheral surface of the coupling body to restrict the position of the locking element. When the female pipe coupling body and the male pipe coupling body are coupled to each other, the locking element projects from the inner peripheral surface of the coupling body to engage an engagement groove of the male pipe coupling member, and the locking element is kept engaged in the engagement groove by being held from radially outside by the inner peripheral surface of the sleeve, thereby undetachably locking the male pipe coupling member to the female pipe coupling member. To decouple the male pipe coupling member from the female pipe coupling member, the sleeve is displaced relative to the coupling body so that an enlarged-diameter portion provided in the inner peripheral surface of the sleeve radially aligns with the locking element, thereby allowing the locking element to be displaced radially outward from the engagement groove of the male pipe coupling member. If, in this state, the male pipe coupling member is pulled out of the female pipe coupling member, the locking element is displaced radially outward by a side wall of the engagement groove and disengaged from the engagement groove.
Thus, when the male pipe coupling member is decoupled from the female pipe coupling member, the locking element is displaced radially outward toward the enlarged-diameter portion of the sleeve. The enlarged-diameter portion has an inner peripheral surface configured to cover the locking element and the locking element retaining hole from radially outside. The inner peripheral surface of the enlarged-diameter portion abuts against the outer top of the locking element to restrict the radially outward displacement of the locking element, thereby preventing the locking element from being completely dislodged from the locking element retaining hole.